Modern agriculture relies on extensive use of fertilizers and chemical pesticides to maintain high levels of production in the face of a rising world population, expanding urbanization and land degradation. However, widespread use of agricultural chemicals has led to environmental problems such as toxicity and eutrophication, resulting in greater interest in alternative approaches to enhancing and sustaining agricultural productivity.
Soil microflora (e.g., bacteria, fungi) can improve plant growth through providing plant-available nutrients (i.e., nutrients in a form that the plant can use), increasing the uptake of mineral nutrients and protecting plants against pests and diseases (Hart el al. 1986; Williams and Sparling 1988; Insam et al. 1991; Srivastava and Singh 1991; Bankole and Adebanjo 1996; Omar and AbdAlla 1998). For example, mycorrhizal association with plant roots is known to enhance growth by assisting with the uptake of phosphorours and other nutrients. Alternatively, a number of plant growth promoting rhizobacteria (PGPR) have been identified which stimulate plant growth by preventing phytopathogens from inhibiting plant growth and development (O'Sullivan and O'Gara 1992; Sivan and Chet 1992; Cook 1993; Glick 1995) or by providing plants with compounds such as fixed nitrogen, phosphate, phytohormones, or solubilized iron from soil (Brown 1974; Kloepper et al. 1988, 1989; Glick 1995).
It has long been known that enhancement of plant growth may be carried out naturally through crop rotation or intercropping with plants belonging to the legume family (e.g., clover, alfalfa). Typically, this can lead to significant increases in the growth and yield of the non-legumep crop. Legume plants form a symbiotic relationship with certain soil microorganisms resulting in the formation of structures called nodules on their roots. Root nodules use plant-derived carbohydrate and the enzyme nitrogenase to convert (fix) atmospheric dinitrogen (N2) gas into a reduced form of nitrogen (N) that plants can use, with hydrogen gas (H2) as a byproduct. As a result, legumes are able to grow without nitrogen fertilizer, an expensive agricultural input that has many environmental costs. Interestingly, only about 25% of the increase in the growth of the non-legume crop can be attributed to improved N nutrition. The remaining 75% of the effect has eluded explanation (Bolton et al. 1976; Hesterman et al. 1986; Fyson and Oaks 1990), although factors that have been proposed include: breaking disease cycles, opening channels for root growth through the hard-pan, and altering the balance of micronutrients in the soil.